Crude oils contain organosulfur, organonitrogen, and polynuclear aromatic (PNA) compounds, which are typically desirable to remove. These compounds are distributed in different distillate cuts at various ratios after the topping process. The heavier the distillates, the higher the level of sulfur, nitrogen, and PNA compounds, and the larger the molecules can typically be. Feedstreams that contain a high level of PNAs can tend to have a relatively low API gravity. Different catalysts and operating conditions may be required in order to achieve predetermined processing objectives.
Catalytic hydroprocessing is an important process in the petroleum refining industry. The purpose of hydroprocessing can vary depending on the feedstream and operating conditions. For example, some objectives can include the improvement of feed quality, abatement of air pollution, the protection of downstream catalysts, and the like. One type of catalytic hydroprocessing is catalytic hydrodenitrogenation (HDN) which involves the removal of nitrogen atoms from organonitrogen compounds. This generally includes hydrogenation of the nitrogen compounds followed by C—N bond cleavage. Thus, the catalyst should generally be able to perform at least two functions, namely hydrogenation and hydrogenolysis. An active HDN catalyst generally balances these two functionalities. The HDN reaction tends to proceed relatively fast with lower boiling feedstreams, but tends to become much slower as the boiling range of the feedstream increases. With higher boiling range feedstreams, e.g., heavy vacuum gas oils and residua, HDN can become more difficult, and complete HDN may not be obtained, even at relatively high severity conditions over the best of present commercially available catalysts. One reason for this can be that heavy heterocyclic nitrogen compounds are generally rather unreactive (or refractory). Another reason can be that intermediate (hydrogenation) reactions can occur that may lead to the formation of nitrogen-containing intermediate species that are more self inhibiting than the parent nitrogen compound. Such HDN intermediates may also inhibit the nitrogen removal of the parent compounds. Further, additional hydrogen would then be consumed to achieve a satisfactory HDN level, and the reaction may also be limited by thermodynamic equilibrium, as the reactor temperature is raised to compensate for catalyst deactivation. Prior hydrogenation of non-nitrogen containing species in the feedstreams (such as arene, aryl, and aromatic ring, or rings, particularly those adjacent to, and adjoined via a nuclear or ring carbon atom with the nitrogen atom to be denitrogenated) may be necessary to achieve a satisfactory level of nitrogen removal. Moreover, at conditions utilized for satisfactory nitrogen removal, other non-nitrogen containing aromatic and/or other unsaturated molecules can also simultaneously be hydrogenated, which can further increase hydrogen consumption over that which is necessary for stoichiometric nitrogen removal.
The following reactions have been found to occur during hydrodenitrogenation of model compounds: (1) HDN of aromatic amines and polyamines (e.g., aniline); (2) HDN of five-membered ring heterocyclic nitrogen species (such as indole and carbazole type compounds), with or without alkyl substituents; and (3) HDN of six-membered ring heterocyclic species (such as quinoline and acridine type compounds), with or without alkyl substituents.
While there are presently commercial processes for removing multi-ring nitrogen heterocycles from hydrocarbon streams, there remains a need in the art for processes that are more efficient and effective through the determination and quantification of the relative concentrations of five- and six-membered nitrogen heterocycles in refinery process feedstreams.